Most HIV infections are spread through sexual intercourse from men to their partners. We have studied the biology of HIV in semen to better understand the requirements of HIV transmission, and to develop clinical and public health interventions for prevention of HIV transmission. Our work suggests that high concentrations of HIV in semen can facilitate HIV transmission. We have demonstrated that gonococcal urethritis causes a tenfold increase in HIV excretion, which can be reversed with antibiotic treatment. Trichomonas vaginalis is a far more prevalent STD than Neisseria gonorrhoeae both in developed and developing countries. Our recent preliminary work suggests that symptomatic inflammatory trichomonas may cause the greatest increase in HIV excretion in semen. In Aim 1 of this Application, we will study the effects of trichomonas on excretion of HIV RNA in seminal plasma. We will determine whether single dose metronidazole therapy (directed at T vaginalis) reduces excretion of HIV in semen. We will examine the relationship between the number of granulocytes in semen and the excretion of HIV. Conversely, we have shown that HIV excretion in semen can be reduced by antiretroviral drugs. Aim 2 of this Application is designed to better understand the relationship between the concentration of selected antiretroviral drugs in semen (measured by Mass Spectroscopy), and their ability to reduce excretion of HIV in seminal plasma. We will examine the phosphorylation of nucleoside analogues in seminal cells. We will also examine the effects of antiretroviral therapy on replication of HIV in seminal cells. We will study the evolution of resistant HIV variants, and try to identify selective pressure(s) in the male genital tract which facilitate resistance. We will compare the "fitness" of HIV variants in semen and blood. These studies will take advantage of a well-developed clinical infrastructure specifically designed to allow us to address complex biological issues. The results of these studies should provide further insight into the regulation of excretion of HIV in semen, transmission of HIV, and to inform public health HIV prevention policies.